Large amounts of fly ash are discharged from power plants in which coal is burned that causes severe pollution and damage to agricultural and natural ecology around the plants. It is therefore now a project that needs to be solved urgently to utilize the fly ash.
Fly ash is also a mineral resource. It generally contains about 15-40% Al2O3 and mostly above 40% SiO2, even above 40% Al2O3 and about 50% SiO2 in high Al fly ash. More than 3 hundred million tons of fly ash are discharged from lire power electrical plant each year in China, wherein 1 hundred million ton is no less than high aluminum fly ash. If resources from high aluminum fly ash that is being abandoned can be fully recovered, more than 30 million tons of Al2O3 can be produced each year, which is far higher than the total production of Al2O3 today in China Bauxite stores in China are only 1/10 of the world average stores. It is therefore significant and important for the sustainable development of an aluminum industry in China to develop and use the resources of high Al fly ash.
The method of recovering Al2O3 from fly ash can be divided into acid methods and alkali methods. With acid processing, damage to SiO2 can be avoided while Al oxide is efficiently recovered from fly ash. But when leaching Al2O3, the disadvantage of acid processing is that a number of soluble impurities such as Fe, Ti, Mg contained in the fly ash are introduced into the solution, so post-treatment must be added; another disadvantage is that apparatus of the acid method needs better acid-corrosion-resistance, so it is rather difficult to make the reaction facility; Another disadvantage regarding the acid process for recovery of Al2O3 from fly ash includes high energy consumption and the necessary cost for environmental protection.
In the 1960's, a soda lime sintering method was used to recover Al2O3 from fly ash in Poland and an experimental plant which produced ten thousand tons of Al2O3 and 100 thousand tons of cement each year was built up there. In the 1980's, Metallurgy Institute of Anhui Province and Hefei Cement Institute in China declared the achievement of recovering Al2O3 from fly ash by sintering limestone and dissolving Na2CO3 and producing cement with the residue. This achievement passed the expert examination in March 1982. The process of recovering Al2O3 from fly ash by soda lime sintering, and producing cement with the residue studied by Building Material Institute of Ningxia Autonomous District was examined by Technology Committee of Ningxia Autonomous District in September 1987. The examination of the project entitled “Industrialization of recovery Al2O3 and producing cement from fly ash”, which was researched and developed by Mengxi high-novel technology group LTD., was held in Inner Mongolia Autonomous Region S. & T. Department in December 2004, and a pilot study of nearly 5000 ton-class was completed by the group itself. But the process of treating high Si and Al-containing fly ash by alkali methods normally has such disadvantages as high complexity, high time consumption, large quantities of processing materials, large investment of facility, high energy consumption and high cost. Furthermore the amount of residue is several times that of fly ash; the market coverage of cement made from the residue is limited; the comprehensive economic efficiency and the level of synthetic utilization are low. All of these factors inhibit the use of alkali methods in synthetic utilization of fly ash.
Today 90% of the Al2O3 in the world is produced by the Bayer Process. But the Bayer Process requires a relatively high Al—Si ratio i.e. ≧7 in raw ore. In order to produce Al2O3 with bauxite of 3<Al—Si-ratio <7, which is economically unfavorable if Bayer Process is directly used, a sintering treatment of ore powder is needed with added alkali and Calcium or complex methods to recover Al2O3; resulting in $20-50/ton higher cost than that of the Bayer Process. The Al—Si ratio of fly ash is generally less than 1, which is far from the conditions on which industry-facilities using the Bayer Process can be used to recover Al2O3 directly.